The invention relates to an electromagnetic actuating unit of a hydraulic directional valve with an armature, which is arranged such that it can be axially displaced within an armature space, and a pole core, which is arranged in a receptacle and which delimits the armature space in a movement direction of the armature.
Such directional valves are used in internal combustion engines, for example, for controlling hydraulic camshaft adjusters. The directional valves are made from an electromagnetic actuating unit and a valve section. The valve section represents the hydraulic section of the directional valve, wherein at least one feed connection, at least one work connection, and a tank connection are formed on this section. By use of the electromagnetic actuating unit, certain connections of the valve section can be connected to each other hydraulically and thus the pressure medium flows can be controlled.
Such directional valves can have a one-part construction, wherein the electromagnetic actuating unit is connected to the valve section that is fixed in place. In these cases, the directional valve is positioned in a receptacle formed, for example, on a cylinder head or on a cylinder head cover and connected via pressure medium lines to the pressure chambers of the camshaft adjuster.
In another embodiment, the electromagnetic actuating unit and the valve section are constructed as separate components, wherein the valve section is arranged radially within an inner rotor of the camshaft adjuster. In this way it is conceivable, for example, to arrange the valve section within a receptacle, which is constructed on the inner rotor, a camshaft, or an extension of the camshaft. In this case, the valve section is arranged coaxial to the camshaft and the inner rotor and rotates together with these parts about the common rotational axis.
In the axial direction to the valve section, the electromagnetic actuating unit is arranged, wherein this unit is fixed in place, for example, to a timing case or the like. The electromagnetic actuating unit controls the axial position of a push rod, which in turn controls the axial position of a control piston of the valve section.
For the use of a directional valve for controlling a camshaft adjuster, the directional valve is normally constructed as a 4/3 or 4/2 proportional directional valve. Such a proportional valve is known, for example, from DE 102 11 467 A1. In this case, the electromagnetic actuating unit is made from a magnetic yoke (pole core), a coil, a housing, an armature, and a connection element, which holds an electrical plug connection used for supplying power to the coil.
The coil and the pole core are arranged coaxial to each other within the housing of the electromagnetic actuating unit. Within the coil, an armature space is formed, which is delimited in the radial direction by the extrusion coating of the coil and in the axial direction on one end by the housing and on the other end by the pole core held in the armature space. Within the armature space there is an armature, which is displaceable in the axial direction and on which a push rod is mounted, which engages through an opening of the pole core and is supported in this opening in the radial direction. The armature, the housing, and the pole core form a flow path for the magnetic flux lines, which are generated by exciting the coil.
The valve section is comprised of a valve housing and a control piston arranged so that it can be axially displaced. The valve housing is constructed as a central screw, which is arranged within an inner rotor of a camshaft adjuster and which locks these in rotation with a camshaft. On the inner rotor, an outer rotor is mounted rotatably, which is in driven connection with a crankshaft in the shown embodiment via a chain drive.
Several pressure medium connections, which are used as feed, discharge, and work connections, are formed on the outer casing surface of the valve housing. The work connections communicate with pressure chambers working against each other and formed within the camshaft adjuster.
In the interior of the valve housing, a control piston is arranged so that it can be axially displaced, wherein the outer diameter of the control piston is adapted to the inner diameter of the valve housing. Ring grooves, via which adjacent pressure medium connections can be connected to each other, are formed on the outer casing surface of the control piston.
By exciting the coil, the armature is forced in the direction of the pole core, with this motion being transmitted to the control piston by means of a push rod attached to the armature. This control piston is now moved in the axial direction against a spring supported on the valve housing, by means of which the pressure medium flow from the feed connection to one of the work connections and from the other work connection to the discharge connection is controlled. In this way, pressure medium is fed to or discharged from the pressure chambers of the camshaft adjuster, by which the phase position of the camshaft relative to a crankshaft can be varied.
In order to guarantee smooth axial displacement of the armature during the operation, a small amount of lubricant is to be fed to the armature space. This is achieved in such a way that a small amount of leakage of motor oil into the interior of the actuating unit is permitted.
During the service life of the actuating unit, the small circulation of lubricant into the actuating unit leads to the result that deposits, for example, of old motor oil or foreign bodies can settle on the running surface of the armature or oil sludge can collect within the actuating unit. This leads to deteriorated response behavior of the actuating unit, higher hysteresis effects, and lower dynamics and can lead up to the seizure of the armature and thus to the failure of the actuating unit and thus the camshaft adjuster.